Liquid crystal display devices have been applied to, for example, watches, calculators, a variety of measuring equipment, panels used in automobiles, word processors, electronic notebooks, printers, computers, television sets, clocks, and advertising boards. Representative examples of types of liquid crystal display devices include a TN (twisted nematic) type, an STN (super twisted nematic) type, and VA (vertical alignment) and IPS (in-plane switching) types involving use of a TFT (thin film transistor). Liquid crystal compositions used in such liquid crystal display devices need to satisfy the following requirements: being stable to external elements such as moisture, air, heat, and light; having a liquid crystal phase in a wide temperature range mainly including room temperature as much as possible; having a low viscosity; and enabling a low driving voltage. In addition, liquid crystal compositions are composed of several to tens of compounds to adjust, for example, the dielectric anisotropy (Δ∈) and/or refractive index anisotropy (Δn) to be optimum to individual display devices.
A liquid crystal composition having a negative Δ∈ is used in vertical-alignment displays and widely applied to, for example, liquid crystal television sets. In all types of driving, there have been demands for low driving voltage, a quick response, and a broad range of operation temperature. In other words, a liquid crystal composition having a Δ∈ with a large absolute value, a low viscosity (η), and a high nematic phase-isotropic liquid phase transition temperature (Tni) has been demanded. In order to determine Δn×d that is a product of Δn and a cell gap (d), the Δn of a liquid crystal composition needs to be adjusted to be in a proper range on the basis of the cell gap. In addition, a quick response is important in liquid crystal display devices applied to television sets or other apparatuses, which generates a need for a liquid crystal composition having a small rotational viscosity (γ1). In particular, an enhancement in a response speed needs a thin cell gap, which has generated needs for both small viscosity and a high Δn in recent years. For such a reason, a liquid crystal composition containing a compound having a fluorine-substituted terphenyl structure has been already developed as disclosed in Patent Literatures 1 and 2.
In order to practically use a liquid crystal composition in a liquid crystal display device, the liquid crystal composition needs not to generate problems with display quality. In particular, liquid crystal compositions used in active-matrix liquid crystal display devices driven by, for example, a TFT device need to have a high specific resistance and high voltage holding ratio. In addition, stability to external stimuli such as light and heat is also necessary. In view of such a circumstance, an antioxidant used for enhancing thermal stability and a liquid crystal composition containing such an antioxidant have been disclosed (see Patent Literatures 3 and 4); however, the effects thereof have been still insufficient. Specifically, since a liquid crystal compound having a large Δn is relatively less stable to light and heat, the liquid crystal composition does not have a sufficiently stable quality.
Furthermore, as liquid crystal display devices have come to be used in a broad range of applications, usage and manufacturing thereof have been greatly changed; in order to adapt to such changes, properties other than known basic physical properties need to be optimum. In particular, a VA (vertical alignment) type and an IPS (in-plane switching) type have become popular as liquid crystal display devices in which liquid crystal compositions are used, and display devices having a very large size (e.g., 50 inches or lager) have been practically used. An increase in the size of substrates has changed a technique for putting a liquid crystal composition between substrates, and a one drop fill (ODF) technique has become mainstream in place of a typically employed vacuum injection technique (see Patent Literature 5); however, dropping a liquid crystal composition onto a substrate generates droplet stains with the result that display quality is degraded, which has become problematic. Moreover, in order to define the pretilt angle of the molecules of a liquid crystal material in a liquid crystal display device and enable quick response, PS (polymer stabilized) liquid crystal display devices and PSA (polymer sustained alignment) liquid crystal display devices have been developed (see Patent Literature 6), and the droplet stains have become more problematic. In particular, such display devices are characterized in that a monomer is added to a liquid crystal composition and that the monomer in the composition is cured; in many cases, the monomer is cured by exposing the composition to ultraviolet light. Hence, addition of a component that is less stable to light leads to a decrease in specific resistance or a voltage holding ratio and causes generation of droplet stains as well in some cases, which results in a disadvantage that is a reduction in the yield of liquid crystal display devices due to defective display.
In terms of these circumstances, a liquid crystal display device which satisfies the following requirements needs to be developed: maintaining properties and functions needed for liquid crystal display devices, such as high response speed; having high stability to, for instance, light and heat; and being less likely to suffer from defective display such as droplet stains and linear afterimages.